This invention relates to a cathode for use in an electrolytic cell, and in particular to a cathode which has a low hydrogen over-voltage when used in the electrolysis of water or brine, eg aqueous alkali metal chloride solutions, and to a method for the preparation of the cathode,
The voltage at which a solution may be electrolysed at a given current density is made up of and is influenced by a number of features, namely the theoretical electrolysing voltage, the over-voltages at the anode and cathode, the resistance of the solution which is electrolysed, the resistance of the diaphragm, if any, positioned between the anode and cathode, and the resistance of the metallic conductors and their contact resistance.
As the cost of electrolysis is proportional to the voltage at which electrolysis is effected, and in view of the high cost of electrical power, it is desirable to reduce the voltage at which a solution is electrolysed to as low a voltage as possible. In the electrolysis of water and aqueous solutions there is considerable scope for achieving such a reduction in the electrolysing voltage by reducing the hydrogen over-voltage at the cathode.
There have been many proposals of means of achieving such a reduction in hydrogen over-voltage.
For example, it is known that the hydrogen over-voltage at a cathode may be reduced by increasing the surface area of the cathode, eg by etching the surface of the cathode in an acid, or by grit-blasting the surface of the cathode, or by coating the surface of the cathode with a mixture of metals, eg a mixture of nickel and aluminium, and selectively leaching one of the metals, eg aluminium, from the coating.
Other methods of achieving a low hydrogen over-voltage cathode which have been described involve coating the surface of the cathode with an electrocatalytically-active material which comprises a platinum group metal and/or oxide thereof as mentioned in for example U.S. Pat. No. 4,100,049 (wherein the coating is appplied from an aqueous solution and then fired), GB 1,511,719 (wherein the coating is applied by electroplating), Japanese Patent Publications Nos. 54090080 (wherein the coating is applied by sinter coating), 54110983 (wherein the coating is applied as a dispersion) and 53100036 and EP 0,129,374 (wherein the coating is applied in the form of salts which are then fired).
In our EP 0,546,714 there is described a cathode for use in an electrolytic cell which has a low hydrogen over-voltage when used in the electrolysis of water or aqueous solutions and which does not depend for its effectiveness on the presence of a coating containing a platinum group metal or oxide thereof The cathode for use in an electrolytic cell disclosed in EP 0,546,714 comprises a metallic substrate and a coating thereon having at least an outer layer comprising a cerium oxide and at least one non-noble Group 8 metal wherein the cerium oxide provides at least 10% and preferably at least 20% by X-ray diffraction analysis of the outer layer.
We have now found surprisingly that cathodes for use in electrolytic cells may be prepared by the physical vapour deposition (PVD) on a suitable substrate of a coating comprising (a) cerium and/or cerium oxide and a non-noble Group 8 metal or (b) platinum and/or platinum oxide and ruthenium and/or ruthenium oxide. Furthermore, we have found that the durability of the cathode may be improved by a subsequent heat treatment.